1. Field of the Invention
The present invention relates to a thermal insulated mold used for resin molding of optical elements, precision components and the like, and to a method for producing the thermal insulated mold.
2. Description of the Related Art
In up-to-date injection resin molding which uses molds for molding resins having fine pattern shapes, thermal insulated molds are useful in order to allow flow of molten resin into fine machined grooves deeply formed on the molds to transfer the shapes of the grooves. With ordinary molds, the heat of the molten resin having an increased temperature to be molded on surfaces of metal molds escapes through metal substrates, resulting in defects in resin molding due to unnecessary reduction in temperature of the resin during molding. However, thermal insulated molds can effectively prevent such molding defects.
Thermal insulated molds have thermal insulating layers having low thermal conductivity and high strength between a mold base material made of metal and a film that forms a transfer surface of resin. For the thermal insulating layers, ceramic materials may be used because of the insulating effect thereof and the strength that is resistant against high molding pressure during resin molding such as injection molding. Among others, thermal insulating layers (hereinafter referred to as zirconia thermal insulating layers) containing plate-shaped members of zirconium oxide (hereinafter referred to as zirconia) sintered body or containing, as a main component, crystal zirconia such as zirconia sprayed films have been employed (see, for example, Japanese Patent Application Publication No. 2004-175112).
In order to meet the requirements for thermal insulated molds for resin molding to allow transfer of complicated and fine configurations, a metal film of a dense amorphous nickel-phosphorous alloy having sufficient machinability is usually formed on a zirconia thermal insulating layer at the upper molding side by electroless plating, which surface layer is then subjected to precision machining.
In case of molding resins, a molten resin generally flows on the surface of a metal film, and thus the metal film has a surface temperature that is the temperature of the molten resin. However in the thermal insulated mold thus manufactured which contains the thermal insulating layer having low thermal conductivity immediately below the metal film, the heat of the molten resin is less transferred to the mold base material made of metal lying below the thermal insulating layer, and thus reduction in temperature of the molten resin is limited. As a result, the molten resin having high temperature can flow into microfabrication patterns such as deep grooves provided on the surface of the mold while maintaining low viscosity, allowing accurate transfer of the pattern configurations.
Metal films as the molding surface of heat insulating molds are generally formed of plated metal films. For example, a mold for optical element molding has been known that is obtained by forming a metal film formed of an electroless nickel-phosphorous alloy plated film which allows facilitated formation of fine patterns by machining on the molding side of a thermal insulating layer and providing microfabrication of transfer patterns by machining on the surface of the metal film (see Japanese Patent No. 4135304).
However, with the above configurations, repetitive heating and cooling during molding of optical elements may cause thermal stress at the border of the thermal insulating layer (ZrO2: coefficient of thermal expansion: 10 to 11×10−6/° C., coefficient of thermal conductivity: 1 to 1.5 W/mK) and the surface electroless nickel-phosphorous alloy plated film (coefficient of thermal expansion: 11 to 12×10−6/° C., coefficient of thermal conductivity: 4.0 to 7.2 W/mK) due to the difference in the coefficient of thermal expansion therebetween, resulting in film detachment.
In order to prevent the detachment, a thermal insulated mold containing an intermediate layer which has the coefficient of thermal expansion similar to both layers, is compatible with both layers in material point of view and has excellent adhesiveness has been proposed. For example, the intermediate layer has been used which is manufactured by high-speed flame spraying of a NiAl alloy having the coefficient of thermal expansion of 13×10−6/K and the coefficient of thermal conductivity of 20 W/mK. Alternatively use of a sprayed film of cermet which is a composite sintered body of metal and ceramic as the intermediate layer or use of a material having the composition that varies along the thickness direction of the film, which has been derived as a result of focusing on the compositions of a thermal insulating layer and a surface processing layer, has been proposed (see, for example, WO 2007/020769).
Also, in order to prevent film detachment in a mold containing a thermal insulating layer formed of a zirconia sintered body due to the thermal stress at the border of two layers resulting from the difference in the coefficient of thermal expansion native to the materials generated during repetitive heating and cooling upon molding, a thermal insulated mold has been known which comprises a porous zirconia sintered body, at a centre of a thermal insulating layer, prepared by sintering zirconia particles; a porous sintered body stacked on the porous zirconia sintered body, that contains mixed particles of nickel metal particles and zirconia particles such that the content of nickel metal particles is inclined so as to be higher at the upper portion of the porous sintered body; a nickel metal sheet material bonded on the porous sintered body with a solver solder; and a nickel-phosphorous plated film layer on the nickel metal sheet material, wherein a porous sintered body is stacked below the porous zirconia sintered body, that contains metal particles having the same compositions as a mold base material (stainless steel) instead of nickel metal particles such that the composition is inclined along the thickness direction of the film (see, for example, Patent Document 4; Japanese Patent Application Publication No. 2010-194805).